Almost alone among U.S. allies, Britain has faithfully met that target. But in the current year, the defense budget will keep pace with inflation only; in real terms, it will not increase at all. And in the next two years, it will fall behind the rate of inflation.

This budgetary restraint comes just as the government is about to embark on a $12 billion program to replace Britain`s aging Polaris nuclear submarines with four American-designed Trident 2 submarines.

Government critics have concluded that the level of spending combined with the cost of the Tridents will not leave enough money for Britain`s conventional defenses.

Liberal Party leader David Steel charged that Prime Minister Margaret Thatcher`s ``delusions of grandeur on Trident will be paid for by the neglect of Britain`s real defenses.`` He added, ``Our soldiers, airmen and sailors will have to make do with antiquated weapons, worn-out equipment, less training and less ammunition.``

If Britain`s conventional defenses do suffer, it undoubtedly will be a source of major concern to Gen. Bernard Rogers, supreme allied commander in Europe.

For the last two years, Rogers has been urging NATO countries to increase their spending for conventional defense to take advantage of new technologies that would offset the Soviet Union`s numerical advantages in Europe.

Futuristic weapons systems needed by NATO, he said, can be funded if nations will raise defense spending by 4 percent annually in real terms for six years.

The loudest criticisms of Britain`s decision to stop raising the defense budget come from the opposition parties. But some members of Thatcher`s own Conservative Party are worried.

The House of Commons Defense Committee said in a report last week: ``With the present policy, there is bound to be a substantial real cut. It was acknowledged in evidence in the clearest terms that the conventional equipment budget is particularly at risk.``

Government officials have denied that conventional programs will suffer, but the committee said testimony by ministers was so ``vague and evasive``

that it was difficult to get at the truth.

The committee said it had developed ``the strongest suspicion`` that there would have to be cancellations or slowdowns in the purchase of new equipment in the 1990s and that old equipment would have to be used beyond its life span.

Kevin McNamara, Labor spokesman on defense, warned in a debate in the Commons last week that Britain could face the ``structural disarmament`` of its forces under present policies.

The Trident program is the major focus of criticism. Labor, the Liberals and the Social Democrats have pledged to get rid of it if they come to power. Britain`s four Polaris nuclear submarines will become obsolete by 1995, and the government argues that Trident offers the only reasonable alternative for maintaining Britain`s independent nuclear deterrent.

For a country with a defense budget of $22.8 billion a year, the Tridents --which cost $3 billion each--clearly are an expensive option. When the administration of President Jimmy Carter agreed to sell the Tridents to Britain, it sought assurances that Britain`s conventional defenses would not suffer.

The government argues that Trident is affordable because costs will be spread over 20 years; on average it will cost only 3 percent of the total defense budget and 6 percent of the defense equipment budget.

The Sunday Times newspaper, in the most extensive review of the new submarine that has been published here, concluded recently that Trident can be accommodated. But it said other elements in the defense budget will suffer.

The peak spending years on Trident, 1989-1992, it said, will coincide with other purchases already planned, for new naval frigates, light attack-helicopters for the army and new radar and sonar systems.

Defense officials concede it may be difficult to carry out a planned $1 billion program to replace the navy`s amphibious forces over the next 10 years. These forces would be needed to put Royal Marines ashore in Norway in event of a European war.

The amphibious program, they said, would be competing for funds with a new fighter plane the Royal Air Force needs to replace Phantoms and Jaguars in the mid-1990s.

Present plans call for a new fighter to be built as part of a five-nation European project, with Britain`s share put at $5 billion to $6 billion.

But this project is in trouble because of disagreements between France and the other partners. If the project collapses, Britain could try to form a partnership with West Germany or it could buy a new fighter from the U.S.